J/ApJ/708/1452 Quadratic F_ij_ force field in l-CH_4_^-^ (Senent+, 2010) ================================================================================ Ab initio characterization of C^-^_4_, C_4_H, and C_4_H^-^. Senent M.L., Hochlaf M. =2010ApJ...708.1452S ================================================================================ ADC_Keywords: Atomic physics Keywords: astrochemistry - ISM: molecules - molecular data - molecular processes Abstract: Using state-of-the-art theoretical methods, we investigate the stable isomers of C^-^_4_, C_4_H and C_4_H^-^. Three of them are relevant for astrophysics and astrochemistry. These computations are performed using highly correlated ab initio methods and the aug-cc-pVXZ (X=T,Q) basis sets. In addition to the linear isomers, we predict the existence of several cyclic and branched forms for these molecules. For all the molecular species of interest here, sets of spectroscopic parameters are determined with perturbation theory, which compare quite well with experiment. For l-C_4_H^-^(X^1^{Sigma}^+^), the quartic force field is computed at the coupled cluster level of theory. This force field is derived from full nine-dimensional potential energy surface generated close to the equilibrium geometry of this anion. Finally, we treat the thermochemistry of the hydrogen attachment and the electron attachment reactions that may lead to the formation of the C_4_H^-^ from either C^-^_4_ or C_4_H. Description: Electronic structure calculations have been achieved with the MOLPRO program suite (MOLPRO version 2002). The search of stationary points and determination of first-order spectroscopic parameters were performed at the coupled cluster including perturbative treatment of triple excitations ((R)CCSD(T); Knowles et al. 1993JChPh..99.5219K) and at the complete active space self consistent field (CASSCF; Knowles et al. 2000JChPh.112.3106K) levels. For these computations, the Dunning's basis sets including diffuse functions were employed (Dunning 1989JChPh..90.1007D; Kendall et al. 1992JChPh..96.6796K). The main group of spectroscopic parameters of the l-C_4_H^-^ anion has been calculated with second-order perturbation theory applied on the quadratic, cubic, and quartic force fields derived from a CCSD(T)/aug-cc-pVQZ nine-dimensional potential energy surface (PES) and the code FIT-ESPEC (Senent 2007ApJ...670.1510S). For CASSCF calculations, the active space was defined considering all the {pi} orbitals. File Summary: -------------------------------------------------------------------------------- FileName Lrecl Records Explanations -------------------------------------------------------------------------------- ReadMe 80 . This file table4.dat 20 187 *(R)CCSD(T)/aug-cc-pVTZ quadratic F_ij_ force field of l-CH_4_^-^ -------------------------------------------------------------------------------- Note on table4.dat: computation with the MOLPRO program suite (version 2002). See Knowles et al. 1993JChPh..99.5219K for (R)CCSD(T) treatment. -------------------------------------------------------------------------------- Byte-by-byte Description of file: table4.dat -------------------------------------------------------------------------------- Bytes Format Units Label Explanations -------------------------------------------------------------------------------- 1- 2 I2 --- i [-7/7] The i component of the force field 4- 5 I2 --- j [-7/7] The j component of the force field 7- 8 I2 --- k [-7/7] The k component of the force field (1) 10- 11 I2 --- l [-6/7] The l component of the force field (1) 13- 20 F8.4 --- Term Most significant term -------------------------------------------------------------------------------- Note (1): When "k" and "l" are zero the force field is anharmonic quartic. When just "l" is zero the force field is cubic and when neither "k" and "l" are zero the force field is quartic. -------------------------------------------------------------------------------- History: From electronic version of the journal ================================================================================ (End) Greg Schwarz [AAS], Emmanuelle Perret [CDS] 22-Feb-2012